A well know characteristic of a microfiltration membrane is the bubble point. The method is based on measuring the gas flow across a liquid-filled membrane as a function of the applied gas pressure. The bubble point is defined as the pressure where the first gas bubble penetrates through the pore. Its value is given by the well known Laplace equation, which for circular pores and full wetting can be written as:
were rp is the pore radius (m), the surface tension between the gas and the liquid (N/m) and P the pressure difference (N/m2).
The bubble point method only gives the size of largest active pore in the membrane. To determine the pore size distribution of a microfiltration membrane, the gas pressure increase can be combined with a continuous measurement of the gas flow across the opened pores in the membrane. This technique is known as Coulter Porometry (named after a manufacturer of the equipment) or Capillary Flow Porometry. For this we use a Porolux™ 1000 (IB-FT GmbH) instrument (Figure 1). Although designed for flat sheet membranes, we adapted our equipment to be able to measure hollow fiber membranes.
Figure 1. Porolux™ 1000 capillary flow instrument.
This method starts with wetting an MF membrane with a pore filling liquid, usually a fluorinated hydrocarbon (Porofil™ or Galpore™). While increasing the gas pressure, starting at very low values, the gas flow across the membrane is measured continuously. At a certain minimum pressure the largest pore opens (the bubble point) and a gas flow can be detected. Further increase of the gas pressure will result in opening of smaller pores, and consequently a higher gas flow is measured. After all pores are opened (‘wet curve’), the gas pressure is decreased to the starting value and the gas flow through all open pores is measured as a function of applied pressure (‘dry curve’). From both curves (see Figure 2), the pore size distribution can be calculated using the Laplace equation and assuming circular pores. This includes also the minimum-, mean flow-, and maximum pore sizes. The mean flow pore size is defined as the pore size where the wet curve crosses the ‘half-dry’ curve (dotted green line in Figure 2).
Figure 2. Schematic gas flow as a function of the applied pressure.
According to the manufacturer’s information, the Porolux™ 1000 can be used for membranes with pore sizes between 13 nm and 500 m. However, this depends very much on the pore filling liquid used, and the mechanical strength of the membrane. Especially for polymeric membranes, the material might not withstand the gas pressure necessary to open up small pores, e.g. when ultrafiltration membranes are tested. For ultrafiltration membranes, an in-house designed and built instrument can be used. This technique is called permporometry, is suitable for polymeric UF membranes (flat as well as hollow fiber shapes), and measures pores in the range 1.5-50 nm. Permporometry was described in detail in our Membrane News Twente edition Winter 2012.